Chapter XVII: The Evolution of Physical Characters 368

(1) Preliminary notice of certain difficulties—cause of
mutation, strength of selection. (2) During the intermediate
period great changes took place, but (3) in general during the
first and second periods the tendency was towards the evolution
and preservation of certain types, (4) which is comprehensible
when the conditions among primitive races are studied. (5) In
the third period more variations have been allowed to persist;
disease becomes of importance. (6) At all times warfare,
migration, and crossing have influenced physical characters. (7)
Factors of possible importance other than selection. (8)
Conclusion.